If it's UV not just a colour bulb is it the kind that gets particularly hot?
If so might be a bit uncomfortable for the nearest side of the tank.
Other than that the only issue is algae if it's on too long.
Simple solution is to put a large book or ringbinder between the tank and the light tho.
Source: I have 2 kinds of UV light one is pretty weak and pretty much a harmless kids toy, the second is CSI grade and will cure nailvarnish and craft resin, it is hot enough to give a sun burn if held over skin too long and came with huge legal disclaimers over what damage it can do to the eyes if you shine it remotely in someone's face. (Not to worry you, I just wanted to get the awareness out there they are varying strength and some can be harmful.)